Advances in the development and improvements of the luminous flux of light-emitting diodes (LEDs) such as solid-state and certain organic or polymer light-emitting diodes have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, light-emitting diodes are becoming increasingly competitive with light sources such as incandescent, fluorescent and high-intensity discharge lamps.
Light-emitting diodes offer a number of advantages and are generally chosen for their ruggedness, long lifetime, high efficiency, low voltage requirements, and the possibility to control the colour and intensity of the emitted light independently. LEDs provide improved performance in comparison to delicate gas discharge lamp, incandescent, and fluorescent lighting systems. Solid-state and improvingly organic light-emitting diodes have the capability to create similar lighting impressions while typically providing greater flexibility than other lighting technologies.
When electrical current through an LED changes rapidly, the heat transfer properties of the device can cause transient temperature gradients exceeding about 3000° C. per cm as shown by Malyutenko et al. in “Heat Transfer Mapping in 3-5 Micrometer Planar Light-Emitting Structures”, Journal of Applied Physics 93(11), 2003, pp. 9398-9400. Rapidly increasing drive currents can generate spatially localized hot spots inside the LED with peak temperatures as high as about 150° C. as shown by Barton et al. in “Life Tests and Failure Mechanisms of GaN/AlGaN/InGaN Light-Emitting Diodes”, SPIE Vol. 3279, 1998, pp. 17-27 in spite of effective cooling of the LED package, which typically reduces the average junction temperature of the LED die. Also temperature gradients induced by rapid transient drive currents in high-flux LEDs can depend on the initial current as shown by Farkas et al. in “Electrical and Thermal Transient Effects in High Power Optical Devices”, Proceedings of the Twentieth Annual IEEE Semiconductor Thermal Measurement and Management Symposium, 2004, pp. 168-176.
As is known, thermally induced excessive mechanical stress inside an LED can lead to premature lumen depreciation. It can also significantly shorten device lifetime due to a number of catastrophic failure modes which can include wire bond fracture or package-die lift-off, for example.
FIG. 1 illustrates how the junction temperature of a green LED changes over time subsequent to switching the drive current from about 400 mA to about 10 mA. These changes can be inferred from the measured forward voltage Uf(t) indicated on the right ordinate of FIG. 1. As can be seen, the rate of change of the junction temperature rate after drive current adjustment can reach several thousand Kelvin per second. As is known, temperature change rates correlate to respective temperature gradients according to the heat transfer equation and depending on the heat transfer properties of the LED. There are a number of applications in which excessive temperature gradients or hot spots may occur.
For example, light-emitting diodes have proven useful for backlighting of liquid crystal display (LCD) panels as used in color television and computer displays as is discussed, for example, by Folkert(s) in “LED Backlighting Concepts with High Flux LEDs”, SID 04 Digest, 2004, pp. 1226-1229, or by Harbers et al. in “LED Backlighting for LCD HDTV”, Journal of the Society for Information Display 10(4), 2002, pp. 347-350, or by Sugiura et al. in “Wide Color Gamut Monitors-LED Backlighting LCD and New Phosphor CRT”, Optical Engineering Society Bellingham, Wash.: Proceedings of Liquid Crystal Materials, Devices and Applications X and Projection Displays X, SPIE-IS&T 5289, 2004, pp. 151-160, or by West et al. in “High-brightness direct LED backlight for LCD-TV”, SID 2003 Digest, pp. 1262-1265.
As is known in the art, display systems are usually designed to receive signals in which information is formatted as a serial stream of data comprising a sequence of frames. Each frame comprises data necessary for rendering a single still picture. In addition, the information signals can comprise data which can identify the beginning or the end of a frame and can aid with synchronizing the display of a single still picture. For example, each frame can comprise a vertical retrace signal. A sufficiently rapid sequence of frames can generate the impression of a flicker free motion picture. The frames may be generated and rendered at a rate dependent upon the desired application of the display. For a number of reasons, certain types of display systems require that backlight LEDs be turned OFF and ON in synchronicity with, for example, vertical retrace signals. The vertical retrace signal time period can be equivalent to several times the thermal time constant of the backlight LEDs which can result in excessive and potentially damaging thermal gradients inside the backlight LEDs which can be detrimental to LED longevity.
For example, as illustrated in FIG. 2, backlighting of a LCD panel can be performed using a technique wherein the backlight LEDs are blanked in synchronicity with vertical retrace signals. In this figure the vertical retrace interval 10 of the video signal 20 for the backlighting of a LCD panel is identical with the OFF time 35 and the ON time 30 of the LED, which is controlled by the LED drive current ID(t).
For example, Yamada et al. in “Sequential-Color LCD based on OCB with an LED Backlight”, Journal of the Society for Information Display 10(1), 2002, pp. 81-85, describes a color video display that utilizes a monochrome LCD with sequentially-enabled red, green, and blue (RGB) LEDs for backlighting. This system is in principle simpler and can likely be more economically manufactured than conventional LCD panels that utilize white backlighting and a matrix of RGB filters, wherein color filter elements are configured for each display pixel. However, each colour of LED can only be energized for about 1.2 milliseconds, or approximately 10% of the time allotted for conventional LCD backlighting. The LEDs must therefore be driven with about ten times the amount of current used for conventional LED backlighting in order to maintain the same LCD screen luminance with the same number of LEDs. As would be readily understood, this operating mode can consequently greatly increase thermal stress on the LED die and its wire bonds, for example.
Prior art drive techniques for high-flux LEDs utilize both analog and digital current control as described by Zukauskas et al. in “Introduction to Solid-State Lighting”, Wiley-Interscience, 2002. As discussed, the most common form of digital control is pulse width modulation (PWM). Both digital and analog controlled LED backlight blanking during the vertical retrace interval comprises switching the drive current from full power to zero, waiting for the duration of the retrace interval, and then switching the drive current back to full power.
U.S. Pat. No. 4,190,836 discloses an LED drive circuit wherein the leading and trailing edges of the drive current pulses are extended by a capacitor in parallel to each LED. The effect that the capacitor has on the drive current is illustrated in FIG. 3, wherein the leading edge 40 and the trailing edge 50 of the drive current are elongated by the charging and discharging of the capacitor. However, for this configuration of a LED drive circuit, the capacitor suppresses higher harmonics only in the total current pulse which may otherwise create high frequency electromagnetic radiation which can interfere with radio frequency signals. The total current is the sum of the LED drive current plus the capacitor current. Consequently, the capacitor reduces the drive circuit load for a repetitively-pulsed LED system and can substantially suppress the generation and emission of higher harmonic electromagnetic radiation. The electronic circuit as disclosed, however, does not affect the transient current through each LED nor does it reduce thermal stress in the LEDs.
Therefore, there is a need for a new method and apparatus that can reduce the thermal stress applied to light-emitting elements during operation, for example during repetitive on and off cycling.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.